Non-ribosomal synthesis allows microorganisms to produce a diverse range of novel compounds including carboxy acids, heterocyclic rings, fatty acids and non-proteinogenic modified amino acids. Small polypeptides are assembled by peptide synthetases just as other compounds, like fatty acids, are linked by other synthetases during synthesis on the ribosome.
Non-ribosomal synthesis provides a pathway of synthesizing compounds which would be expensive or unobtainable using synthetic chemical methods. Nonribosomally synthesised peptides share certain characteristics. These small bioactive peptides are usually between 2 and 50 amino acid residues long and possess potent biological activities. Most examples of these compounds are also highly resistant to physical and chemical degradation making them ideal for use as oral therapeutics. The valuable products of microbial nonribosomal peptide synthesis include the immunosuppressant cyclosporin A and antibiotics such as penicillin, gramicidin S, vancomycin, cephalosporin, and surfactins.
Such compounds are synthesized by complex secondary metabolism pathways involving polyketide synthases (PKS), non-ribosomal peptide synthetases (NRPS) and fatty acid synthases (FAS). These enzymes are activated by phosphopantetheinyl transferases (PPTs). PPTs activate carrier proteins that are essential for PKS, NRPS and FAS activity. PPTs convert inactive carrier proteins to their active, cofactor-bearing holo-forms via transfer of the essential prosthetic 4′ phosphopantetheine moiety from co-enzyme A (CoA) (FIG. 1).
Cyanobacteria constitute a rich source of secondary metabolites with the majority being derived from PKS, NRPS and FAS. Due to their broad intergeneric distribution of integrated enzyme systems, cyanobacteria provide a source of many uncharacterised amino acid activating and modifying peptide synthetase modules. Consequently freshwater and marine cyanobacteria have been screened for bioactivity revealing natural products with novel biological applications and clearly implicating cyanobacteria as a rich source of potentially useful compounds.
Analysis of cyanobacterial non-ribosomal peptide, polyketide and fatty acid synthesis pathways has revealed unforeseen biochemical structures displaying the potential for novel products from cyanobacteria. However the strains of cyanobacteria which synthesize natural compounds are usually strains associated with slower growth or complicated culture requirements. Thus species which produce natural products may be excluded in culture-based screening. Furthermore current natural product screening relies on organism propagation. As less than 1% of microorganisms are estimated to be culturable, however, the bulk of potential bioactivities present in nature is not detectable. Alternatively, time-consuming organic syntheses have been used to create a modified natural product. Although molecular methods may yield the genetic information, recombinant expression in a suitable host organism is required to allow the isolation and production of novel compounds.
The ability to heterologously express and biochemically characterize recombinant proteins and biosynthesis pathways remains a significant problem with respect to cyanobacteria.